This invention relates to a composite hard material and to a cell for forming a composite hard material and to methods for forming the composite hard material as well as articles formed from hard materials.
Our earlier international patent application Nos. PCT/AU88/00058, PCT/AU85/00271 and PCT/AU92/00127 disclose methods of forming polycrystalline diamond composite hard material (PCD) and polycrystalline cubic boron nitride hard material (PCBN). International application PCT/AU88/00058 also discloses a cell for forming the hard material.
Current commercial production of PCD""s and PCBN hard materials is performed in triaxial, belt and girdle or similar apparatus at pressures of at least 50 Kb. These apparatus yield products with the geometric configuration of wafers or discs that are optionally bonded to carbide substrates during the fabrication process. These shapes are not optimum for certain hard-rock and mining operations and further machining and cutting to shape of the discs or bonded composites can be needed. This adds to final costs.
A first aspect of the invention concerns forming shaped pieces of hard material which can be used without major EDM finishing. Conventional techniques for forming composite hard material generally yield a disc or cylinder shaped piece of material. The disc or cylinder is not perfectly symmetrical and generally the finished piece is machined from the disc or cylinder by laser cutting or an EDM process. For example, if it is desired to form tips for a mining machine, a cylinder of hard material is produced and the cylinder is then subject to EDM processing to form the dome or conical shaped tip. The EDM processing ensures that the hard material is formed into the required shape. EDM finishing adds significantly to the time taken to produce the finished product and also the cost of the finished product.
This first aspect of the invention may be said to reside in a method for producing a hard composite material, including:
locating a charge of material in a mould having at least one shaped cavity having the shape of a finished product;
subjecting the mould and the charge of material to high temperature and pressure to form a composite hard material; and
maintaining the charge subject to essentially quasi-hydrostatic pressure during the application of pressure and high temperature.
By maintaining the charge under quasi-hydrostatic pressure and at low temperature gradient, the final shape of the product is determined by a predicted mould form which results in the required product size and shape and therefore product of the required shape can be produced in the forming process without the need for substantial EDM finishing. This therefore reduces costs and time taken to produce a finished article from the composite hard material.
Preferably the method further includes maintaining a low temperature gradient radially and axially across the charge during formation of the composite hard material to reduce internal stresses that might result in delamination.
Preferably the charge is maintained at a substantially low temperature gradient during formation of the hard material.
Preferably the mould having the shaped cavity is formed from graphite or similar material and a plurality of shaped cavities are formed in the mould.
Preferably the shaped cavities are dome shaped or cone shaped for the formation of tips for a mining machine.
Preferably the shape composite hard material is formed in a cell as described below.
The second aspect of the invention may be said to reside in a cell for forming a composite hard material, including:
a cell wall defining a central region for receiving the composite hard material;
the cell wall having;
(a) a ductile wall member;
(b) a heater for providing heat energy when electrical current is applied to the heater;
(c) means for maintaining a substantially low temperature gradient throughout the central region in which composite hard material is formed;
(d) a water barrier wall arranged inwardly of the ductile wall member and heater; and
(e) a barrier layer arranged inwardly of the water barrier wall for preventing intrusion of material from which the water barrier wall is formed into the composite hard material during formation of the composite hard material.
By maintaining a substantially low temperature gradient within the central region of the cell the material produced has lower residual stress that resits delamination and fracture.
Preferably the means for maintaining a substantially low temperature gradient comprises a metal reflector formed from high melting point metal material arranged between the ductile wall member and the water barrier wall for reflecting radiant energy from the heater and also conducting heat energy axially towards ends of the cell.
Preferably the means for maintaining a substantially low temperature gradient may also includ graphite spacers within the central region in which the hard material is located to alter the current density and improve thermal conductivity axially from the central region thereby aiding in the reduction of the thermal gradient caused by heater damage and by thermal conductivity.
Preferably the water barrier wall comprises a glass wall which also serves to enhance even pressure distribution and create a quasi-hydrostatic environment within the cell.
Preferably the heater comprises a high melting point material such as graphite or molybdenum.
Preferably the barrier layer comprises titantium foil or similar material.
Preferably the ductile wall is formed from talc or other material which provides good thermal and electrical insulation. The ductile wall also assists in creation of the hydrostatic environment within the cell to ensure even pressure distribution over the hard material during formation of the composite material.
This aspect of the invention also provides a method of forming a composite constituents into a cell as described above and applying electric current to the heater to heat the charge and applying pressure to pressurise the charge to thereby form the composite hard material.
A third aspect of the invention may be said to reside in a method for forming a composite hard material, including:
locating a charge of constituents of the composite material in a cell for the formation of the composite hard material;
increasing the temperature of the charge and subjecting the charge to pressure to form the composite hard material; and
reducing the pressure quickly after formation of the composite hard material whilst maintaining a high temperature.
Preferably the pressure is elevated to about 25 Kb during the formation of the composite hard material and the pressure is pressure is reduced rapidly from about 25 Kb to ambient pressure in about four minutes.
Preferably the temperature of the charge is raised to approximately 600xc2x0 C. and pressure is increased to 6 Kb. The temperature and pressure are then increased simultaneously to a temperature of 1050xc2x0 C. and a pressure of 10 Kb. The temperature is then held and pressure is raised to a maximum pressure of about 25 Kb. At 25 Kb pressure, the temperature is slowly raised to the sintering temperature and held for a predetermined period after which the temperature is again ramped slowly to 800xc2x0 C. Once this temperature is stabilised, the pressure is then reduced to the ambient pressure over a period of about 4 minutes. At about 0.5 to 1 Kb, the pressure release is slowed and the temperature ramped to ambient slowly over about 8 minutes.
A third aspect of the invention concerns the structure of the hard composite material articles.
This aspect of the invention provides a composite hard material article, including:
a body having a variable microstructure, said microstructure being represented in an outer surface and an interior core, the body being formed from composite hard material including hard particles; and
the body having a surface layer at the outer surface which has a higher hard particle content than the interior core thus forming a more closely packed hard particle network in the surface layer than in the interior core of the body.
This aspect of the invention also provides a method of forming a hard material article, including the steps of:
loading a charge of hard composite material including hard particles into a mould, the mould substantially defining the outer surface shape of the article;
applying temperature and pressure to the charge of material in the mould in such a manner so as to form at the outer surface a surface layer which includes a hard particle content greater than that of an interior core portion of the article thus forming a more closely packed hard particle network in the surface layer than in the interior core of the article.
The formation of the outer surface having the higher hard particle content provides significant advantages in that the outer surface of the article is provided with an increased hardness and improved wear performance compared to articles formed from the same material wherein the outer surface and interior core have a substantially homogenous hard particle content.
The composite hard material may be polycrystalline diamond composite hard material in which the hard particles are formed from diamond, or polycrystalline cubic boron nitride hard material wherein the hard particles are formed from the cubic boron nitride.
Preferably the application of temperature and pressure to the charge of material includes the steps of:
locating the mould containing the charge into a cell;
initial application of pressure and temperature causing load to be transmitted to the mould located in the cell to cause initial axial shortening of the cell to in turn cause pressure increase resulting in a reduction in the height of the mould;
further increasing pressure and temperature to create a quasi-hydrostatic pressure environment within the cell to create relative uniform compression of the material;
further increasing pressure so that the material is progressively densified firstly by compaction of the surface that is in contact with the mould and then progressively inward to the centre of the charge;
causing a pressure gradient to be generated within the charge thus producing comparatively lower compaction towards a central region of the charge; and
further increasing temperature to allow the bonding agent to react with the hard material particles and form a bonding matrix.
In one embodiment the charge has:
a first mixture having hard material particles and a second mixture having finer hard material particles;
and the method further includes packing the first mixture into the mould so as to be adjacent the mould surface to define a cavity which will define the interior core of the article;
filling the cavity with the second mixture containing the finer hard particles; and
placing the mould including the first and second mixtures into the cell and applying pressure and temperature to form the article.
In a further embodiment of the invention the hard material particles are diamond and the bonding agent is silicon. An additional quantity of silicon in the form of a wafer with similar diameter to the mould is placed on the flat face of the mould. During the final high temperature step the silicon wafer melts and penetrates the composite further reacting with the diamond particles to form additional silicon carbide bonding matrix.
In a further embodiment penetration of the body by additional molten silicon is not uniform but is preferentially concentrated in the core. The composite thus produced exhibits a microstructure in which the domed surface layer is relatively enriched in diamond and the core is relatively deficient in diamond and possesses an enhanced matrix content. This microstructure confers practical advantages in improving tip lifespan for mining operations.
In another embodiment of this invention the charge has a first mixture having hard material particles and a second mixture having fewer hard material particles and the method further includes packing the first mixture into the mould so as to be adjacent to the mould surface to define a cavity which will define the interior core of the article.
As polycrystalline diamond composites posses relatively low tensile strength and low fracture resistance, the above aspects of the invention enhance and improve these material properties. The laminated/layered feature caused by the diamond concentration effect of the moulding process and cycling technique offer benefits with regard to tool performance in certain mining applications. Due to the higher diamond content of the outer surface of the tool, increases in ultimate wear resistance will be achieved thus prolonging tool life. Other benefits of the diamond layering referred to above will be an improvement in impact resistance and higher bulk strength, thus tool chipping and other damage caused as a result of violent contact or shock when in service will be reduced.